In well drilling apparatus of the types referred to as `top drive` and `side drive` equipment, the drill string is driven by a powered rotary stem which is connected to the upper end of the string and advances downwardly with the string as the drilling operation progresses. When it becomes necessary for any reason to remove the string from the well, the string is broken down into a series of stands of pipe each usually consisting of three individual lengths of pipe which remain threadedly connected together and are handled and stored in the rig as a unit.
U.S. Pat. No. 4,658,915 issued Apr. 21, 1987 to G. A. Goris et al on "Easy Break-Out Tool Joint And Method" shows a device which is connectable between the powered stem of a top drive or side drive mechanism and the upper end of a drill string, and which is intended, upon reverse rotation of the stem, to break the threaded connection to the string without at the same time unscrewing the individual lengths of pipe from one another. To attain this purpose, the connector of that prior patent includes a tubular body which is threadedly connectable to the upper end of the drill string and which carries a separately formed ring having a shoulder adapted to bear against an upper shoulder of the string to limit advancement of the threads together and transmit torque to the string. Cam surfaces are provided between the tubular body of the device and the shoulder ring to retract the ring axially relative to the body upon rotation of the body through a limited angle relative to the ring in a joint breaking direction. This retraction is intended to relieve the tight engagement between the shoulders and enable the device to be disconnected from the upper end of the string with a break-out torque less than its make-up torque. In utilizing this device, there has been provided between the cam surfaces a lubricant having less friction than the lubricant normally employed between successive lengths of pipe in a particular stand, to encourage initiation of the camming action.
A problem encountered with this prior device resides in its tendency, under some operating conditions, to break the threaded joint between the connector and the string unintentionally and unpredictably during normal handling of the equipment. The break-out torque required to disconnect the drive unit from the string is not accurately predeterminable, and may be so light that the joint can break and release the string at times when detachment is not desired. Further, the camming action has the effect under some circumstances of causing only a partial loosening of the joint, without complete disconnection, and this partial loosening may upon subsequent rotation in a make-up direction cause the joint to tighten to a greatly excessive torque. The tendency for only partial loosening results from an undesirable variation in the resistance offered to unscrewing rotation of the device. As the connector is turned in a joint breaking direction, the resistance first decreases as the cam surfaces move relative to one another, and then at the end of the camming action the resistance to further unscrewing rotation increases and tends to retain the parts against complete separation.